1. Field of the Invention
This invention relates generally to a system and method for automatically determining the concentration of contaminants and, more particularly, to a system and method for providing automatic quantification of critical ionic species on a printed circuit board, printed circuit board assembly following cleaning or a no-clean wave soldering process.
2. Discussion of the Related Art
High capacity manufacturing processes that rapidly produce a large number of printed circuit board (PCB) assemblies generally include machines that attach the electrical components onto the PCB, and machines that later solder the components in place on the board. After the electrical components have been positioned on the PCB assembly, but prior to the PCB assembly being subjected to the soldering step, a flux is applied to the PCB assembly by a flux application apparatus within a soldering machine. Such soldering machines may take on many different forms, such as wave fluxing or foaming machines, open spray fluxing machines, or mist systems. The soldering flux cleans the conductive traces on the PCB assembly in order to improve intermetallic bonding, and thus provide better electrical connections.
Traditional fluxes were generally resin based fluxes that sometimes left residues and contaminated the PCB assembly. Modern fluxes are generally either alcohol based or water based, where the alcohol or water makes up about 95% or more of the flux solution. The remaining 5% or less of the flux solution is a suitable acid and/or other chemicals depending on the particular flux application. For discussion of a particular low residue soldering flux of this type, see for example U.S. Pat. No. 5,004,509 issued Apr. 2, 1991 to Bristol.
With the traditional resin based fluxes, the PCB assembly was generally required to be cleaned following the soldering process to remove flux residue that may contaminate the PCB assembly and affect its operation, particularly the interconnection of the electronic components. With alcohol or water based fluxes, it was not necessary to clean the PCB assembly to remove flux residue because the majority of the flux would evaporate, and the residues would be small. However, evaporation of the fluxes on the PCB assembly does leave behind some residue, some or most of which may be ionic in nature. What is meant by ionic is that the residues may be conductive. Various ionic residue of this type are known in the art.
These ionic residues can have a harmful affect on the operation of the PCB assembly. Particularly, the ionic residue on the PCB assembly can cause electron migration across signal traces on the PCB assembly, resulting in a transfer of charge that is unintentional. Such a transfer of charge may ultimately reduce the useful life of the device, thus affecting its reliability. Other, more serious problems can also occur, such as component failure as a result of this ionic residue.
Currently, a manual process is used to quantify the ionic residue on a PCB assembly in order to monitor the PCB manufacturing process to control such residue. One known testing method is referred to as extraction, and includes the process of bringing loosely bound ionic contaminates into a solution of a known volume of deionized water. Particularly, a PCB assembly is removed from the process line, and placed in a Kpak bag, known to those skilled in the art. A graduated cylinder is used to measure a known volume of deionized water, and the measured quantity of water is placed in the Kpak bag. The Kpak bag is then placed in a hot water bath for a predetermined period of time so as to cause the ionic residue on the PCB assembly to be removed and dissolved in the water. A syringe is then used to extract a certain amount of a water sample from the Kpak bag and manually inject the sample volume into an ion chromatograph such as the DX-100 Ion Chromatograph, commercially available from Dionex of Sunnyvale, Calif., for analysis. The analysis process includes injecting the solution from the Kpak bag into a lour loc fitting located on a front panel of the ion Chromatograph. AI-450 software is used to control the operation of the ion chromatograph. After inputting a sample name and a dilution factor, the operator initiates the analysis cycle by invoking a start command. The ion chromatograph then gives the quantification of the constituent matter in the solution which can then be correlated to a specific amount of contamination per unit area on the PCB assembly.
In order to compare the solution to a reference so as to determine the ionic constituent residue, the system must be calibrated at start up, and whenever new eluent is added to the ion chromatograph. The calibration frequency is directly related to use, and is typically performed on a weekly basis. Generally, system calibration requires three successive runs with a known calibration standard.
A known calibration sequence for the testing system is given as follows. First, a volumetric flask for each calibration point is obtained, and each flask is filled with deionized water to about one-half of its total volume. Then, a desired volume of a known calibration standard is added to each flask, and the remaining volume of the flask is filled with the deionized water to obtain three final solutions of known concentrations. After the calibration standards are prepared, three successive sample runs using the calibration standards are performed. Following the successive calibration runs, the calibration is checked for linearity with a Calplot program that tests the linearity between each level of calibration. This relationship is calculated for each ion, and the calibration where all values are above 0.999 are deemed acceptable.
As is apparent from the above description, the known process for determining the quantity of ionics reside on a PCB assembly is not well suited to the manufacturing floor. This is because the above described process is very operator interactive and requires a higher than normal amount of training and responsibility. The calibration is complex and critical, and is generally outside the scope of operator responsibilities. Also, manual sample volume measurements and calibration standard mixing draws immediate question to the accuracy and repeatability of the process. The AI-450 software interface used in the control of the operation of the ion chromatograph is structured for a laboratory environment, and requires a higher than normal level of computer literacy.
What is needed is a method and apparatus for determining the quantity of ionic residue on a PCB assembly that is relatively easy to use and is capable of being performed on the manufacturing floor near the PCB assembly stations. It is therefore an object of the present invention to provide such a process and apparatus.